Long-term antiorthostatic hypokinesia stimulates storage of osmotically inactive sodium in the human body

Changes in the water and sodium balances and in the states of the fluid compartments of the human body observed in experiments performed with healthy subjects exposed to long-term (120 days) antiorthostatic hypokinesia (ANOH) were analyzed. A hypothesis was suggested that the normal dietary consumption of sodium could be associated with the accumulation of osmotically inactive sodium in the body of a healthy person (independently of changes in the total water content). The results agree with the assumption that considerable amounts of osmotically inactive sodium may be stored in the human body. This hypothesis was confirmed by the inversion of the correlation between the cumulative sodium balance and the total water content of the body found both in the group-averaged data and in individual data. This nonsmotic sodium accumulation may take place not only during deviations from its normal consumption, but also during its regular dietary supply. Accumulation of sodium in these stores and its depletion are not associated with any significant changes in the volumes of body fluids. Infradian rhythmic changes in the sodium balance observed in some subjects exposed to the long-term ANOH, which were not caused by any periodic external influences, indicated the existence of a specific mechanism regulating the sodium content of the body. This mechanism must be significantly more inert and less precise than the fast regulation of the volume, osmolality, and ionic composition of extracellular fluids.